1. Field of the Invention
The present invention relates to the production of precipitated silicas and, more particularly, to a novel process for producing synthetic precipitated silicas and silicates having new and improved physical and chemical properties.
2. Description of the Prior Art
As known in the art, finely divided amorphous precipitated silicic acid products and certain zeolitic type alumino silicates may be prepared by the acidulation of an aqueous silicate solution with an acid or a salt of the acid, such as aluminum sulfate. Such products are commercially available being sold e.g., under the trademarks "Zeo"; "Zeolex" and "Arogen" by the J. M. Huber Corporation. Specific examples of these products as well as methods for their preparation are disclosed in U.S. Pat. Nos. 2,739,073; 2,843,346 and 3,582,379.
While the nature or characteristics of the above discussed precipitated silicic acid (sometimes herein referred to as silicas) and silicates depend, in part, on the chemistry of the silicate solution (specifically the SiO.sub.2 /Na.sub.2 O ratio of the silicate) as well as the reaction conditions employed (precipitating pH, etc.), prior to the present invention such silicas were characterized by the following properties: high structure, high wet cake moisture content, high oil absorption, low valley abrasion, high surface area and low pack density. In this regard, and due in part to the properties such as high oil absorption, high surface area, etc., the silicas have been widely and successfully used as reinforcing materials in rubber, in paints, in the manufacture of paper, as moisture conditioners and the like.
However, the generally speaking for the moment, the high wet cake moisture content is disadvantageous in that the drying and filtration rates are decreased thus increasing the overall cost in the production of the final product. For example, in the conventional production of silicic acid materials as defined above the wet cake moisture content of the product (following filtration of the precipitated reaction mass) is approximately 82%. This means that there can be recovered only 18 parts of dry silica from 100 parts of wet cake.
Further, and very importantly, the low abrasiveness and high refractive index of known silica and silicates renders them unsuitable for many uses. For example, it is well known that conventional synthetic precipitated silicas are unsuitable as polishing and abrasive agents in toothpaste compositions. See German Pat. No. 974,958; French Pat. No. 1,130,627; British Pat. No. 995,351; Swiss Pat. No. 280,671 and U.S. Pat. 3,250,680. In this regard, it is disclosed in U.S. Pat. No. 3,538,230 that known amorphous silicas such as precipitated silicas, pyrogenic silicas and aerogels are unsuitable for dentifrice use because they show substantially no cleaning ability on human teeth because of their initial small particle size and because of the ease in which they break down into small particle sizes which result in poor cleaning ability.
Further, and in more detail, conventional silicas and amorphous precipitated alumino silicates, such as "Zeolex" and "Arogen", cannot be used for a clear gel toothpaste because of their high refractive index (1.55) and because they lack the needed abrasive and polishing characteristics when added to the toothpaste base composition. Clear gel toothpaste contains a high percentage of abrasive and polishing agent in the toothpaste formula. The major function of the abrasive and polishing agent is to remove stains, food debris and bacterial plaque from the human tooth surface. Ideally the polishing agent should provide a maximum cleaning action at acceptable abrasion levels and must be compatible at high loadings of 15% up to 50% with other toothpaste formula ingredients. Thus known silicas and alumino silicates are unsuitable for clear gel toothpastes, (such as the product sold under the Trademark "Close-up" by Lever Brothers) because they cannot be added at high loadings of 15% and above in a typical toothpaste composition. Because of their high oil absorption, high sorption characteristics and high refractive index (1.55) known precipitated pigments thicken up the dentifrice composition and impart undesirable opacity to the base paste resulting in an unacceptable product. In summary of the above, precipitated silicas and silicates cannot be used in conventional and clear gel dentifrice compositions because such products result in unacceptable toothpaste consistencies and do not possess the acceptable abrasive and polishing characteristics needed for use in dentifrice compositions.